Delay in Embryonic Development, Uterine Epithelial Changes and Implantation due to Maternal Treatment during Preimplantation Period*

Abstract

The hormonal requirements for ovum implantation vary among species; at least in the mouse and rat, progesterone and estrogen, of which production and secretion are controlled along the hypothalamo‐pituitary‐ovarian axis, are indispensable for the initiation of an implantation. In fact, implantation of the blastocyst in these species is delayed or prevented by not only ovariectomy, but also by: hypophysectomy, lesion of the median eminence or a treatment of the maternal organism with an agent that interferes with this endocrine control system. The preimplantation developmental events such as cleavage, blastocyst formation and shedding of the zona pellucida and gestational changes of the uterine luminal epithelium are delayed under a few conditions causing a delayed implantation. Experimental data suggesting that uterine changes leading to the formation of a ‘receptive’ endometrium are hormone‐dependent are accumulating. Thus, the observed delay in changes in the uterine epithelium may be attributed to the alteration of the endocrine control. In contrast, it is difficult to determine how the delay in embryonic development occurs. Among the conceivable explanations are: the direct effect on embryos (ova) of implantation‐delaying factors, a mechanism synchronizing embryonic development with the uterine change or disturbance of possible hormonal control over the embryonic development.